This invention relates to antennas and more particularly to a broadband horn antenna useful for direction finding or for feeding a reflector.
The corrugated horn antenna, known also as the scalar horn antenna, is a conical or square horn antenna with coaxial corrugations or slots formed in the horn wall along axially spaced planes that are transverse to the axis of the horn. This antenna has many advantages including a circularly symmetrical radiation pattern essentially free of side lobes and a substantially constant beamwidth. The useful bandwidth of this corrugated horn, however, is approximately 1.7:1 which limits its applications. For example, there are microwave receivers currently available which may be tuned over frequency ranges of 8-12 GHz and 12-18 GHz, respectively, so that two receivers are employed to cover the 8 to 18 GHz band. It is advantageous for many reasons to have two such receivers share a single antenna but to accomplish this, the antenna must have an operating bandwidth of at least 2.25:1, i.e., it must have acceptable performance characteristics over this band.
Efforts to extend the bandwidth of the corrugated horn antenna have included forming the horn with broadband slots such as partially dielectrically loaded slots, tapered slots, or ridge loaded slots, the latter being described in a paper entitled "The Ring Loaded Corrugated Waveguide" by Y. Takeichi et al published in IEE Transactions on Microwave Theory and Techniques, December 1971, pages 947-950. While such horn constructions have resulted in some bandwidth improvement, the radiation pattern nevertheless still deteriorates at the upper end of 8 to 18 GHz band so that the antenna is unacceptable for use in high performance receiving systems operating over this band.